The coating of textile materials and leather with polyurethane systems is prior art. A distinction is drawn here between aqueous polyurethane dispersions and solvent-containing systems.
The aqueous polyurethane systems cover a wide field of use and have the advantage of needing essentially no volatile organic substances. Because of their necessarily hydrophilic character, corresponding coatings, however, have lower water stability than the corresponding polyurethane coatings produced from organic solutions, since the hydrophilizing groups remain in the coating film. For production of coatings with good water stability, therefore, polyurethane systems composed of organic solvents are preferable over aqueous systems.
Polyurethane coatings based on organic solvents are highly appreciated among users on account of their hardness, elasticity and stability. Such systems are produced by reacting diisocyanate with linear macrodiols (polyether diols, polyester diols or polycarbonate diols) to give a prepolymer and then adjusting them to the required molar mass by reaction with diamines as chain extender. Because of the structure consisting of the hard urea segment and the soft macrodiol segment, the polyurethane urea solutions thus produced have a tendency to associate and crystallize out of organic solution. As a result, good processibility and good properties of the polyurethane coating are no longer assured. More particularly, it is problematic to produce organic solutions from polyurethane ureas having a sufficiently high molecular weight to achieve sufficiently good mechanical properties of the coating obtained without precipitation of the polyurethane ureas out of the solvents.
The use of solvent mixtures recommended in the art for the prevention of this crystallization, on account of the growth in knowledge of toxicology, has now become controversial. For aliphatic polyurethane ureas, DE-A 3 134 112 and DE-A 2 457 387, and also U.S. Pat. Nos. 3,912,680 and 3,904,796, use solvent mixtures of, for example, considerable proportions of aromatic hydrocarbons with alcohols, including mixtures of toluene or xylene with isopropanol or tert-butanol.
On the basis of recent findings in toxicology, toluene is a hazardous substance which should be used only to a limited degree in solvent mixtures for polyurethane ureas for textile coating, and should be used only with observation of relatively complex measures. The same applies to xylene, which has been classified as irritant and harmful to health. The amide- and urea-containing solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone or tetramethyl urea that have likewise been customary to date in textile coating are likewise solvents that have been classified as harmful to health, and therefore complex and costly safety measures have to be observed when using them.
DE-A 102008032779, WO-A 2006/089648 and WO-A 2007/082665 disclose specific polyurethane solutions which are suitable as textile coatings and must always contain γ-butyrolactone as co-solvent. However, γ-butyrolactone as co-solvent is also undesirable in some applications. Polyurethane urea solutions for coatings in general are additionally also known from U.S. Pat. No. 3,609,112. In addition, polyurethane urea coatings based on organic solvents play an important role in the production of synthetic leather by coating of textiles. Particularly, what is called the coagulation process, as described in “New Materials Permeable to Water Vapor”, Harro Träubel, Springer Verlag, Berlin, Heidelberg, New York, 1999, ISBN 3-540-64946-8, pages 42 to 63, is a frequently employed method. This involves coating a textile substrate with a polyurethane solution in dimethylformamide (DMF). Subsequently, the coated substrate is guided through a plurality of DMF/water baths with increasing proportions of water. Contact with water results in coagulation of the polyurethane and formation of a microporous film. The products produced by this method are especially used as high-grade synthetic leathers, for example the CLARINO and ALCANTARA synthetic leather brands.
A great disadvantage of the granulation process is that it is necessary to use large amounts of DMF, which is classified as hazardous to health. Therefore, it is necessary to take particularly complex measures to safeguard workers, and it is necessary to dispose of or reprocess large amounts of DMF/water mixtures.
There is therefore a constant search for alternative polyurethane solutions suitable for the coagulation process in uncontroversial organic solvents.